1. Field of the Invention
The present invention relates to a heat-sealable and peelable film for polystyrene containers.
It also relates to a process for sealing a polystyrene container by means of said film and to the heat-sealed polystyrene container. Finally, it relates to the use of a composition for producing a heat-sealable and peelable film for polystyrene containers.
2. Description of the Prior Art
Heat-sealable and peelable films are employed on a large scale for temporarily closing containers containing, for example, food products. During the use the consumer tears away the said film. Thus, these heat-sealed films must exhibit a number of characteristics which will be described concisely below.
The film must permit a leakproof closure of the container. It will be recalled that the characteristics of the sealing are the combined application of heating and of pressure, making it possible to melt the sealable layer and to "squash it" over the edge of the container to form the welding strip. In addition to a certain softening or squashing of the upper layer, a good sealing also requires a softening or distortion of the container. In a known manner, the sealing is performed with the aid of heating jaws (over a usual range from 100.degree. C. to 220.degree. C.), at an appropriate pressure generally from 0.1 to 1MPa, for short times of the order of a second.
Such a film must also meet the peelability criteria which are defined by a peel strength needed to open the cap or lid. This peel strength is, in fact, representative of the tight adhesion between the sealable layer and the substrate. A strength of between 200 g/cm and 600 g/cm is usually synonymous with a good closure-opening compromise.
Furthermore, the industry today demands packaging in which all the constituents (films, decorations, caps) are recyclable with the main material. This increasingly requires the replacement of various materials such as paper, aluminum, laminates or coextrudates with adhesives of the "hot melt" type or those containing solvents.
Many heat-sealable films have already been proposed in order to satisfy the abovementioned criteria.
Thus, document EP-A-406,681 describes a multi-layer heat-sealable film consisting of a layer of styrene/butadiene/styrene copolymer and of another layer based in particular on polyamide, it being possible for this film additionally to comprise an intermediate layer consisting, for example, of impact polystyrene (that is to say a polystyrene reinforced with a small quantity of polybutadiene), it being possible for the adhesion between the different layers to be improved by an adhesion promoter based on an ethylene/vinyl acetate/maleic anhydride copolymer.
Document JP-A-78-31934 [CA106 (26): 215186h] describes a heat-sealable film for polystyrene containers, containing a mixture of 15% of ethylene acrylate copolymer and 70% of a styrene/butadiene/styrene (SBS) block copolymer and 15% of a polystyrene. The disadvantage of the presence of SBS in a large quantity is due to the fact that the film loses some of its elasticity because of the degradation of SBS in the course of time (insufficient drawability), and this also entails an increase in its heterogeneity. Furthermore, this document is intended for the applications of films which can be closed again after opening (repeelability).
Document JP 86-166671 [CA106 (24): 197630s] describes a heat-sealable film for polystyrene containers, comprising a mixture of an ethylene and acrylate copolymer and a styrene/butadiene/styrene block copolymer.
Note should also be taken, in a different field, of document EP-A-412,503, which describes an adhesive composition containing a mixture of ethylene/vinyl acetate copolymer, of polystyrene, of polyacrylate, of a styrene/butadiene/styrene copolymer and of an ethylene/alpha-olefin copolymer. These compositions are not intended for the production of heat-sealable film.